[clang-scan-deps] Fix contention when updating TrackingStatistics in hot code paths in FileManager. (#88427)
`FileManager::getDirectoryRef()` and `FileManager::getFileRef()` are hot code paths in `clang-scan-deps`. These functions are updating on every call a few atomics related to printing statistics, which causes contention on high core count machines.
After this patch we make the variables local to the `FileManager`.
In our test case, this saves about 49 sec over 1 min 47 sec of `clang-scan-deps` run time (1 min 47 sec before, 58 sec after). These figures are after applying my suggestion in https://github.com/llvm/llvm-project/pull/88152#issuecomment-2049803229, that is:
```
static bool shouldCacheStatFailures(StringRef Filename) {
return true;
}
```
Without the above, there's just too much OS noise from the high volume of `status()` calls with regular non-modules C++ code. Tested on Windows with clang-cl.
39ed3c68e5